The DX7 and DX7s have different batteries, you want one that fit's the DX8.
Don't send it in, the nimh packs are
just look for a lipo. There is one offered by HH for $69, or you can get a turnigy for $17.95. I've also seen some on ebay for about $30.
I have to charge mine often, and it drops voltage from 5.4 to 5.1 in the first flight for me...